Structure characterization of the membranes by XPS analysis

other hand, the bands corresponding to the carboxylic acids and CF2 functional group indicated a weak absorption.

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Fig. 2.5. XPS survey scan of (a) untreated PVDF membrane, (b) PVDF membrane treated with argon plasma (treatment: ± 4.0 kVp-p, 180 s) and (c) PVDF-g-PAA membranes (treatment: 20% (v/v) AA, 70°C, 30 min).

Two peaks at BEs of 286 and 291 eV attributable to C1s and a strong peak at BE of 688 eV attributable to F1s were observed in the XPS survey scan spectrum of the non-treated hydrophobic PVDF membrane (Fig. 2.5a). The XPS survey scan spectrum of the PVDF membrane treated with argon plasma consisted of peaks at BEs of 286, 291, 534, and 688 eV, attributable to C1s, C1s, O1s, and F1s, respectively (Fig. 2.5b).

On the other hand, the result of the XPS measurement onto PVDF-g-PAA membrane consisted of peaks at BEs of 285, 289, and 533 eV, attributable to C1s, C1s, and O1s, respectively (Fig. 2.5c). It shows that successful grafting of PAA on the plasma-treated membrane was performed since the fluorine content on the surface decreased substantially after grafting.

Atomic composition for the PVDF membrane surfaces modified by argon plasma was estimated from relative intensities of C1s, F1s, and O1s high resolution spectra. Results of XPS analysis for the F/C and O/C atom ratios of the PVDF membrane surface treated with argon plasma are summarized in Table 2.1.

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Table 2.1

Atomic ratios of PVDF membrane surface exposed to argon plasma.

Plasma treatment Atomic ratio Defluorination Voltage

[± kVp-p]

Flow rate [L min^-1]

Exposure time [s]

F/C O/C [%]

- - - 1.01 0.01 0

4.0 5 60 0.50 0.16 50

4.0 5 180 0.38 0.20 62

4.0 5 300 0.31 0.20 69

2.8 5 180 0.58 0.14 43

4.0 5 180 0.38 0.20 62

5.0 5 180 0.32 0.21 68

4.0 4 180 0.42 0.19 58

4.0 5 180 0.38 0.20 62

4.0 6 180 0.43 0.18 57

The weak peak at BEs of 533 eV on the survey spectrum of untreated PVDF, attributable to O1s signal, confirmed that the PVDF surface was partially oxidized.

However, the O/C atom ratio for the untreated PVDF was 0.01, so it was considered to be low level. All plasma-treated PVDF membrane surfaces showed lower F/C atom ratio than the untreated PVDF surface, and higher O/C atom ratio. In other words, a reduction in fluorine intensity occurred together with an increase in oxygen intensity and relative increase in the carbon. These changes indicate that the plasma exposure led to defluorination including dehydrofluorination and oxidation reactions on the PVDF membrane surfaces.

In this work, I investigated the influence of defluorination and oxidation on the plasma treatment time, applied voltage and flow rate of argon gas. Firstly, defluorination estimated from the F/C atom ratio increased by 50% in 60 s, 62% in 180 s, and 69% in 300 s at ± 4.0 kVp-pcompared to untreated PVDF. Secondly, defluorination resulted in 43% at ± 2.8 kVp-p, 62% at ± 4.0 kVp-p, and 68% at ± 5.0 kVp-p in 180 s compared to untreated PVDF. Therefore, with increasing the treatment time and voltage, defluorination was gradually in progress. It was also confirmed that defluorination was occurred even at the minimum voltage of ± 2.8 kVp-p when plasma could be formed.

Likewise, oxidation reaction estimated from the O/C atom ratio was also progressed by increasing the rate of the plasma treatment time and applied voltage, however oxidation was approximately the same beyond 180 s or ± 4.0 kVp-p. Finally, as a consequent of evaluating the effect of flow rate of argon gas on defluorination and oxidation, defluorination was 58% at 4 L min^-1, 62% at 5 L min^-1, and 57% at 6 L min^-1 for 180 s compared to untreated PVDF. Similarly, oxidation was the most progressed in 20% at 5

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L min^-1, whereas it was almost at the same level between 4 L min^-1 and 6 L min^-1. Collectively, it was concluded that the most effective flow rate of argon gas was 5 L min^-1 to abstract fluorine atoms from untreated PVDF and produce reactive sites on the membrane surface for further modification.

To discuss defluorination, dehydrofluorination and oxidation reactions in detail, I investigated the data of narrow scanning C1s and O1s core level spectra. Fig. 2.6 shows C1s and O1s spectra for untreated PVDF membrane, PVDF membrane treated with argon plasma and PVDF-g-PAA membrane. The decomposed peaks were illustrated in dotted lines.

Fig. 2.6. High resolution XPS spectra of C1s (a) and O1s (b) of untreated PVDF membrane. The peak numbers in these figures corresponds to the Peak No. of Table 2.2.

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Fig. 2.6. High resolution XPS spectra of C1s (c) and O1s (d) of PVDF membrane treated with argon plasma (treatment: ± 4.0 kVp-p, 180 s). The peak numbers in these figures corresponds to the Peak No. of Table 2.2.

Fig. 2.6. High resolution XPS spectra of C1s (e) and O1s (f) of PVDF-g-PAA membrane (treatment: 20% (v/v) AA, 70°C, 30 min). The peak numbers in these figures corresponds to the Peak No. of Table 2.2.

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Table 2.2 summarizes the BEs and functional groups from the results of high resolution XPS analysis.

Table 2.2

Summary of high resolution XPS scan results for untreated PVDF membrane, PVDF membrane treated with argon plasma (treatment: ± 4.0 kVp-p, 180 s) and PVDF-g-PAA membrane (treatment: 20% (v/v) AA, 70°C, 30 min). When it is considered that some functional groups are included in each peak, these groups are identified by a number.

Values represent the percentage associated to each or sum of bond type. Note that (-) denotes 0%. The underlined C or O means the objective carbon or oxygen.

Peak No. BEs [eV]

Functional groups Untreated PVDF

Plasma-treated PVDF

PVDF-g-PAA

1 285.0 -CH(COOH)-CH2-CH(COOH)- - - 43.0 2 285.4 CH-COOH - - ^28.5

3 286.4 (1) -CF2-CH2-CF2- (2) -CH2-CF=CH- (3) -CH2-CF=CH2

(4) -CH2-CH(-OOH)-CH2-

(1) 48.7 (1,2,3,4) 49.2

-4 287.-4 -CF2-CH(-OOH)-CF2- - 16.7

-5 288.-5 (1) -CF2-CHO (2) -CH2-CFH-CH2-

- (1,2) 13.5

-6 289.1 CH-COOH - - ^28.5

7 289.5 -CH2-CF(-OOH)-CH2- - 3.2 -

Peak No. BEs [eV]

Functional groups Untreated PVDF

Plasma-treated PVDF

PVDF-g-PAA

8 290.9 (1) -CH2-CF2-CH2- (2) -CH2-CFO (3) -CH=CF2

(1) 51.3 (1,2,3) 17.5

-9 532.2 (1) CH-COOH (2) -CH2-CFO

(3) -CF2-CHO

- (2,3) 2.8 (1) 47.4

10 533.5 (1) CH-COOH (2) -CH2-CF(-OOH)-CH2- (3) -CF2-CH(-OOH)-CF2-

- (2,3) 23.6 (1) 52.6

11 534.0 C-OOH - 73.6

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The C1s high resolution spectrum of untreated PVDF membrane surface assigned to two distinct peaks at BEs of 286.4 eV due to CF2-CH2-CF2 :No. 3 and 290.9 eV due to CH2-CF2-CH2 : No. 8. Plasma-treated PVDF membrane and PVDF-g-PAA membrane showed complex C1s spectra. The underlined C or O in these linkages means the objective carbon or oxygen, and each number of decomposed peaks corresponds with peak number in Table 2.2.

The C1s high resolution spectrum of plasma-treated PVDF membrane was decomposed into five peaks as shown in Table 2.2. The five peaks appeared at BEs of 286.4, 287.4, 288.5, 289.5 and 290.9 eV, which were assigned to CF2-CH2-CF2, CH2-CF=CH, CH2-CF=CH2 and hydroperoxide (CH2-CH(-OOH)-CH2) groups (No. 3) ; hydroperoxide (CF2-CH(-OOH)-CF2) (No. 4); carbonyl (CF2-CHO) and CH2-CFH-CH2

groups (No. 5); hydroperoxide (CH2-CF(-OOH)-CH2) (No. 7); and CH2-CF2-CH2 and carbonyl (CH2-CFO) and CH=CF2 groups (No. 8), respectively. The composition in Fig.

2.6(c) indicates surely that CF2 carbons were modified into CFH, C(-OOH)and CFO carbons in the plasma exposure. On the other hand, the C1s high resolution spectrum of PVDF-g-PAA membrane was decomposed into characteristic three peaks attributable to AA graft polymerization. The three peaks observed at BEs 285.0, 285.4, and 289.1eV, which were individually assigned to -CH(COOH)-CH2-CH(COOH)- (No. 1);

CH-COOH (No. 2); and carboxy (CH-COOH)group (No. 6).

Likewise, the O1s high resolution spectrum of plasma-treated PVDF membrane was curve-fitted with three peaks at BEs of 532.2 eV for carbonyl (CH2-CFO) and aldehyde (CF2-CHO) groups (No. 9), 533.5 eV for hydroperoxide (CH2-CF(-OOH)-CH2, CF2-CH(-OOH)-CF2) groups (No. 10) and 534.0 eV for hydroperoxide (C-OOH : No.

11) . On the other hand, the O1s high resolution spectrum of PVDF-g-PAA membrane was integrated with typical two peaks at BEs of 532.2 eV for carboxy group (CH-COOH : No. 9) and 533.5 eV for carboxy group (CH-COOH : No. 10).

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